We should include .sln files in the binlog when building slnfs

### Issue Description

.sln files are not included in the .binlog when building .slnf files.

### Steps to Reproduce

Create a .sln (dotnet new sln)
Create a .slnf (like [here](https://devblogs.microsoft.com/visualstudio/load-only-the-projects-you-need-with-solution-filters/))
msbuild .slnf /bl
Open the binlog

### Expected Behavior

binlog contains your .sln

### Actual Behavior

.sln is absent

### Analysis

### Versions & Configurations